uORB: Added a new uorb_shutdown function that is called during normal shutdown procedures. It will only

call into a new UORB COMMUNICATOR ICHANNEL shutdown interface if it has been configured, otherwise it
does nothing. This allows ICHANNEL implementations to pass on a shutdown indication to a remote processor.
Implemented the shutdown interface in the muorb module for VOXL flight controllers.

platforms/common/apps.cpp.in

@@ -2,6 +2,7 @@
 #include <px4_platform_common/time.h>
 #include <px4_platform_common/posix.h>
 #include <px4_platform_common/log.h>
+#include <uORB/uORB.h>
 
 #include "apps.h"
 
@@ -43,6 +44,7 @@ void list_builtins(apps_map_type &apps)
 int shutdown_main(int argc, char *argv[])
 {
 	printf("Exiting NOW.\n");
+	uorb_shutdown();
 	system_exit(0);
 }